1. Field of the Invention
The present invention relates to a semiconductor laser device, and more particularly to a semiconductor laser device used as a light source for optical communications and optical disk devices.
2. Description of the Related Art
High-capacity, recordable and portable optical disk systems are growing rapidly in popularity and use as external storage for personal computers, etc. Semiconductor laser devices having high optical output efficiency and good optical and temperature characteristics must be developed to satisfy the requirements of small or portable optical disk systems. On the other hand, with the spread of public networks using optical fibers, there is an increasing need to transmit a large amount of information at low cost. Increasing the information transmission rate so as to meet such a need requires a semiconductor laser device having high optical output efficiency.
A known example of a conventional semiconductor laser device is a DFB laser device configured such that: a p-InP spacer layer having a film thickness of 200 nm is disposed on an MQW-SCH active layer; a diffraction grating made up of a GaInAsP layer is disposed on this spacer layer; and a p-InP first cladding layer having a diffraction grating buried therein is disposed on the GaInAsP diffraction grating disposed on the spacer layer (see, for example, paragraph [0024] and FIGS. 1 and 2 of Japanese Laid-Open Patent Publication No. 2001-320125).
However, the above DFB laser device has interfaces between the p-InP spacer layer and the diffraction grating made up of the GaInAsP layer and between the diffraction grating made up of the GaInAsP layer and the p-InP first cladding layer. At these interfaces are formed heterojunctions of the first kind, which have low energy levels for both types of carriers (electrons and holes). The bandgap energy of the diffraction grating made up of the GaInAsP layer is lower than those of the p-InP spacer layer and the p-InP first cladding layer.
Therefore, both electrons and holes are likely to accumulate in the GaInAsP layer constituting the diffraction grating since its bandgap energy is low. When the carrier concentrations of the accumulated electrons and holes each have reached approximately 1×1018 cm−3 (hereinafter 1E18 cm−3), the electrons and holes combine within the diffraction grating layer and, as a result, an reactive current which does not contribute to the laser oscillation flows, causing the problem of increased threshold current of the laser oscillation and reduced luminous efficiency.